Application of colloidal suspensions for polishing advanced materials has become an exceedingly critical aspect of final part formation for the glass and microelectronics industries. Silica and alumina colloids are commonly utilized for polishing various microelectronic materials (copper, tungsten, silicon, etc.), and ceria colloids are frequently used for high quality glass surface processing.
Critical issues in generating high quality optical surfaces for silicate-based glasses, such as fused silica, include the removal of surface and subsurface damage remnant from various preliminary grinding processes and the generation of relatively small topographical features with a Ra typically below 5 .ANG.. For primarily mechanically-oriented abrasive such as zirconia and alumina, the final surface finish tends to be dictated by the size and morphology of the abrasive grains. For chemical-mechanical polishing abrasives such as cerium and iron oxides are likely used, since residual damage on the glass surface is lessened since the abrasives are heat treated to be a hardness comparable or softer than that of the glass. For this reason, cerium oxide has been the abrasive of choice for the majority of glass polishing applications since the 1940s.
The development of colloidal silica as a polishing abrasive is two fold. Colloidal silica has a spherical morphology and varied particle size (typically 20-50 nm diameter) which minimizes scratches in softer materials. By mixing colloidal silica in aqueous solution for polishing materials such as aluminum and silicon, the surface of the metal substrate hydrolyses and permits the abrasive nature of the colloidal silica to remove the reaction layer, while minimizing interactions with an underlying surface.
For glass polishing, pH is most commonly adjusted to be acidic in order to prevent dissolution of the glass surface. This procedure has resulted in part from the fact that the different glasses will corrode and form reaction layers in widely varying fashions. By polishing glass at a relatively low pH, the glass surface does not corrode, but rather has the opportunity to chemo-mechanically interact with the cerium oxide abrasive and promote removal in a controlled manner.